AN EXTRA 12 commuters can crowd into each Sydney train carriage in peak hour, and the system should be progressively broken up and sold off, work prepared for Infrastructure NSW's state infrastructure strategy recommends.

The strategy, released on Wednesday, does not call for the privatisation of RailCorp, the state-owned operator of Sydney's trains. But the plan it proposes for Sydney's trains was informed by analysis advocating their gradual privatisation, while avoiding large-scale investments by trying to get more out of the existing network.

The future of Sydney's train system is the main point of difference between the Infrastructure NSW report and a separate transport plan developed by the state's transport department, Transport for NSW.

The Transport for NSW report advocates another harbour crossing, which would add more than 60 per cent capacity for services through the suburbs and the city.

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The crossing would be built after the north-west rail link is finished in about 2019, but planning for it would start immediately.

The report, released by Infrastructure NSW yesterday, also looked at how to get more out of the train system while spending little on infrastructure.

One of its recommendations is for more people to fit on peak-hour trains. Peak hour services already run up to 180 per cent of capacity at times.

''It is not unreasonable to assume that trains can carry 50 extra passengers in the short-term (two years), and 100 extra in the medium-term (five years),'' the report says. ''Split between eight coaches, these are increases of only six and 12 passengers per carriage.''

And it says single-deck trains should be introduced between Chatswood and Strathfield to increase capacity over the harbour. This idea was adopted in the Infrastructure NSW plan.

A lot of the supposed benefits of the Interfleet plan would come from privatisation, which has been ruled out by the Transport Minister, Gladys Berejiklian.

The report recommends first privatising the Eastern Suburbs and Illawarra line, which runs largely separate from the rest of the train system. Within five years, other lines should be broken off.

The report also advocates running trains from the north west rail link to Central via Epping and Strathfield, and not only to Chatswood.

This would reduce overcrowding on the north shore when the north-west rail line is finished in 2019. However, Ms Berejiklian, has refused to consider this possibility.